The carboxyl group (symbolized as COOH) has both a carbonyl and a hydroxyl group attached to the same carbon atom, resulting in new properties.

Carboxyl groups frequently ionize, releasing the H from the hydroxyl group as a free proton (H+), with the remaining O carrying a negative charge. This charge "flip-flops" back and forth between the two oxygen atoms, which makes this ionized state relatively stable. (Hydroxyl groups sometimes ionize momentarily, but the resulting ionic forms are not stable and the ions immediately rejoin.)

Molecules containing carboxyl groups are called carboxylic acids and dissociate partially into H+ and COO–.

Carboxyl groups are common in many biological molecules, including amino acids and fatty acids.

The figure to the left illustrates acetic acid, a simple 2-carbon acid found in vinegar. Watch as the carboxyl group ionizes and the resulting ionized group is stabilized by the negative charge flip-flopping between the two oxygen atoms.